The PIR sensors we can buy from ebay might make a very usable security system for those in need. They use passive infrared to detect motion so could be used for people or animal incursion detection using a minimum of power and also require no controller as they have a direct voltage output with adjustable sensitivity.

Seems like a good idea. Plus you can get fairly cheap piezo sirens and flood flights (LED preferably because of the lower power usage) and put those on the output of the relays. If we wanted to get more complex with it, we could have a 'duino or other microcontroller on it with one of those little 433mhz or NRF24L radios on it and have a little base station with LEDs that light up on a board with a floor plan of a building or yard to indicate a person or otherwise being in that area. Still fairly cheap, a 5 sensor setup like this could easily be done for under $50 if you source the parts right. Ill put on my list of things to tinker with, which is growing at a quite alarming rate